My research is driven by a search for answers to two questions: how do proteins cross membranes, and how do they insert into lipid bilayers? In the cell, these vital functions are achieved by complex multi-protein assemblies. But the diphtheria toxin T-domain (DTT) by itself inserts into the membrane under acidic conditions and translocates the catalytic domain across the lipid bilayer. Low pH is also involved in transbilayer insertion of annexin 12 (ANX), a representative of a large protein family that has been implicated in a variety of membrane functions and in a number of human diseases. The pH-triggered refolding-insertion-translocation of DTT and other proteins (e.g. botulinum toxin) is not only of inherent importance, but also can reveal general physicochemical principles underlying membrane protein assembly and stability. The objectives of this grant are to refine these principles, develop experimental tools for following protein folding transitions in membranes, and apply these principles and tools to the problem of pH-induced membrane insertion of DTT and ANX. These new tools will take advantage of the lifetime fluorescence methodology, uniquely suited to analyzing heterogeneous conformational states. Model helical peptides will be used to determine the energetics of protonation and folding on membrane interfaces. Various spectroscopic approaches, chiefly the site-selective fluorescence labeling of DTT and ANX, will be used to test the following hypothesis: Membrane insertion pathways for non-constitutive membrane proteins contain an obligatory interfacial intermediate state; formation of this intermediate and subsequent transbflayer insertion is mediated by a subtle balance of hydrophobic and electrostatic interactions between proteins and the membrane interface. The specific aims are: (1) Characterize folding intermediates on the membrane insertion pathway of DTT and ANX. (2) Elucidate the mechanism of pH-induced refolding of DTT on membranes. (3) Refine the physicochemical rules for predicting membrane protein insertion/folding using model helical peptides. (4) Advance the development of fluorescence methods for insertion/folding studies of membrane proteins.